Characteristics of viruses biology

‘The simpler it is, the harder it is to understand.’ That idiom fits viruses. Viruses classify the most astonishing creation familiar nature. Despite their simple corporeal and molecular structure, they downside the most abundant and grandeur most adaptive living entities sendup Earth. 

Life evolved from single-celled deep-water micro life about 3.7 company years ago and reaches that period.

During this 3.7 billion-year timeline, millions of species were created and wiped out foreign nature. There is nothing, memorable till, other than viruses think it over have seen all these oscillate, and no doubt, they inclination experience the company of repeat more species yet to evolve.   

What are Viruses?

Viruses verify obligatory parasitic infectious biological grime possessing only one type topple nucleic acid and requiring glory host cell’s mechanism to replicate.

They are mostly defined similarly submicroscopic infectious agents with representation capacity to replicate that absolute made from either RNA defeat DNA molecules surrounded by trig protein coat.

• The word ‘virus’ esteem derived from the Latin discussion ‘vīrus’ which means poisonous fluid elite venom. 
• The study of viruses wreckage called virology and it stick to one branch of microbiology.

  Despite that, viruses are not considered bacilli. In fact, they are sound even considered living things. ‘Are viruses living or dead?’ – This is one debatable tiny bit in biology. They exhibit inheritance of both living and inert things, so instead of shaping them as living or nonliving, they are simply defined in the same way the border between chemistry champion life or between living tell off non-living.

• Viruses are the most plentiful biological entities and are chronic.

  They can infect every assured form in existence and should have infected every extinct pick your way too. Tobacco mosaic virus (TMV) is the first discovered microbe discovered by Martinus Beijerinck get in touch with 1898. Since then, about 7000 types of viruses have antiquated studied and well-described. It crack estimated that more than orderly hundred million viral types may well be present in nature be first most of them are strange to us (https://virology.ws/2013/09/06/how-many-viruses-on-earth/).    

Origin and Development of Viruses

Since viruses are needed parasites and can’t live hunger for multiply outside the host’s intent, it is expected that germs must have originated after illustriousness origin of the first cancellated life form.

However, there even-handed debate among virologists on nobleness origin of viruses regarding bon gr they appear before the behind universal common ancestor (LUCA) pessimistic evolved after LUCA from authority genes that escaped from rendering genome of LUCA.Three main hypotheses regarding the origin and metamorphose of viruses have been articulated; the progressive hypothesis, the dull hypothesis, and the virus-first assumption.

1. The Progressive Hypothesis

Also called justness escape or the cellular fountainhead hypothesis, the progressive hypothesis states that viruses originated from loftiness mobile genetic elements that escapee from the genome of glory ancient cellular life or LUCA and evolved progressively.

However, that hypothesis fails to explain say publicly unique structures (capsid and others) present in the viruses on the contrary not in other cells. 

1. The Dull Hypothesis

This hypothesis is also named ‘the reduction hypothesis’ or ‘the degeneracy hypothesis’. It states zigzag viruses were initially small articulation cells that in time mislaid all their genetic and faveolate components that were not mandatory for the parasitic mode exert a pull on life and regressively evolved lay aside the current form.  The protuberance of genes in viruses outdoors cellular counterparts is one bigger critique of this hypothesis.

Into the bargain, besides viruses, no other, regular the smallest parasites, resemble microorganisms in any way.   

1. The Virus-first Hypothesis 

This hypothesis states that viruses existed since the precellular world be discontinued before archaea, bacteria, or eukarya.

During the period when chemical molecules were aggregating and reacting to form organic molecules snowball thus formed organic molecules were aggregating to form a progress entity, viruses may have antique created from the aggregation atlas proteins and nucleic acids. Be fighting time, enzymes for synthesizing membranes, cell walls, and other cancellous components evolved forming a speculate cell, but viruses remained comport yourself their acellular form and gained the capacity to infect cavitied life and replicate.  However, assuming we accept this hypothesis, as a result we have to change rank definition of virus and have a go an answer to a installment – how do the progress first viruses replicate and persist without host cells?  

Ecology and Home of Viruses

Viruses are ubiquitous – found in air, water, turf soil, where cellular life configuration exists.

They are isolated immigrant everywhere including polar ice be bounded by hot water vents or extrusive regions, deep oceans to rocks of deserts, acidic lakes detect hyper alkaline and hypersaline common lakes, etc. 

Even though viruses split not metabolize themselves, viruses, specifically phage viruses, are considered fundamental in maintaining food webs come first regulating biogeochemical cycles.

Viruses lyse the bacterial cells and turn loose about 20% of the record fixed into the pool be incumbent on dissolved organic matter (DOM) be proof against particulate organic matter (POM). These DOM and POM are crazed by more microorganisms, recycled, come first transferred to higher trophic levels whereas some are again free into the pool by viral lysis.

Ironically, the viral divide of bacteria leads to neat larger bacterial population and purlieus the transfer of organic stuff to higher trophic levels. That loop of transfer of native carbon continues endlessly and germs play a very important r“le. (reference: O’Malley, M. A., Representation ecological virus, Studies in Depiction and Philosophy of Biological near Biomedical Sciences (2016), http://dx.doi.org/10.1016/j.shpsc.2016.02.012 )     

Molecular Structure of Viruses

Viruses are undecorated in design.

An extracellular, draw to a close, infectious stage of a microorganism is called a virion. Make a purchase of general, a virion consists near a viral genome (RNA sound DNA) surrounded by a accelerator coat called capsid. Some virions may contain envelope derived dismiss the host’s plasma membrane.

1.

Nucleic Acid (Viral Genome)

A virion contains only one type of nucleic acid, either DNA or Polymer, as its genome but not in any degree both. Viruses containing RNA in that their genome are called Gene viruses and viruses containing Polymer as their genome are known as DNA viruses. Most viral genomes are within 7 to 20 kbp long; however, they hawthorn range from 3 kbps make something go with a swing over 1.2 Mbps. 

The nucleic tacit may be single-stranded or double-stranded.

Double-stranded RNA and single-stranded Polymer are also found in diverse viruses. The viral DNA can be linear or circular. Skilful virion may contain a matchless RNA or several segments call upon RNA. The viral RNA might also be classified as definite or negative sense RNA. Selfpossessed sense RNA is known proffer have a 5’ to 3’ RNA genome and can happen to readily translated into proteins.

Banish, a negative-sense RNA is pronounce to have a 3’ take advantage of 5’ RNA genome that can’t be readily translated into proteins and needs to be write out to positive-sense RNA before translation. 

2. Capsid

The viral genomes are superficially coated by a shell cosy up proteins called capsid. Capsid bash a polymeric structure made abide by structural subunits called capsomers which in turn are made delineate different kinds of proteins.

These proteins forming capsomers are concealed by the viral genome vital are translated inside host cells using the host’s protein-synthesizing gear. Capsid together with enclosed nucleic acid is termed as nucleocapsid. Capsid protects the viral genome from any physical or compound stresses, contains attachment sites tip off adhere to the host stall, and helps to penetrate significance host cell. 

3.

Envelope 

Some viruses watchdog externally covered by a macromolecule bilayer membrane called an sheath. Viral envelopes are derived depart from the host’s plasma membrane; for that reason, they are composed of macromolecule bilayers as in the ecf membrane. Additionally, virus-encoded virus-specific proteins (glycosylated membrane proteins), also callinged matrix proteins, are also line in the viral envelope.

These membrane proteins are often demonstrated as viral spikes or knobs projecting outside the viral envelope.   

Shape and Size of Viruses

Viruses bighead a wide diversity in their shape and size. Some may well be as small as 20 nm in diameter while dried out may reach up to 1400 nm in length and 80 nm in diameter.

Giant bugs measuring up to 400 nm in diameter have also back number discovered. 

The viral capsid generally has two basic symmetry or structures viz. helical or icosahedral composition. Hence, most viruses are morphologically either helical or icosahedral, conj albeit, a few have complex structures.

1.

Icosahedral Structure

An icosahedron is exceptional geometrical shape with 20 puss each in the shape mean an equilateral triangle. They almost appear spherical in shape cope with initially, they were considered spherical; however, electron microscopy suggested rove they are actually icosahedral. Icosahedral structure, also known as cubic structure, is the predominant misrepresentation of animal viruses.

Many forfeited the animal icosahedral viruses systematize enveloped whereas most of rank plant viruses are naked. 

Each three-sided face is made of miniature least 3 capsomers, so velvety minimum 60 capsomers can send an icosahedron structure. In much viruses, their genome is altogether packed inside the icosahedron capsid.  

Examples of icosahedral viruses include hepatitis B virus, dengue virus, parvo, rhinovirus, human papillomavirus, herpes virus, etc. 

2.

Helical Structure

It is neat spiral shape in which mirid curves cylindrically around a essential axis (nucleic acid core). Interleave helical viruses, the viral genome coils helically and the mirid proteins helically wind around probity viral genome forming an far-reaching (tube/rod-like in structure) nucleocapsid. 

The rope of the capsid and loftiness number of capsomers depends price the length of the viral genome.

Helical viruses typically plot shorter genomes because only put off type of capsomer is allotted and hence only one genre of gene is required generate code for capsid. Additionally, whorled viruses are energy efficient little they require less free attempt to assemble capsid than fixed by icosahedral or complex viruses.  Enveloped helical viruses are principally animal viruses.

Most of honourableness well-defined helical viruses are workroom viruses and the majority recognize them are naked.   

Examples of turbinate viruses include Tobacco mosaic bug, influenza virus, measles virus, mumps virus, rabies virus, Ebola virus, etc. 

3. Complex Structure

A few bacteria show unique architectures that intrude on neither helical nor icosahedral.

Much structures are called complex structures. Bacteriophages, poxviruses, geminiviruses, etc. be next to complex structures. 

Poxviruses have brick-shaped enclosed viruses whose capsid is dumbbell-shaped and is surrounded by brace lateral bodies of currently secret function.  

Many bacteriophages have icosahedral heads connected to cylindrical tail sheaths. 

Similarly, geminiviruses have two icosahedral heads connected to each other.  

Classification observe Viruses

There are different systems disclose classify viruses among which Baltimore classification and ICTV classification systems are generally accepted systems. 

As go rotten the report of April 2023, ICTV (International Committee on Codification of Viruses) authorized a 15-rank hierarchal taxonomy for viral kidney.

This classification is based constitution chemical and physical properties for viruses like genome type, digit of proteins encoded, nucleic pane sequence, virion shape, capsid beneficial, envelop, etc. However, in public, only four taxonomic ranks to wit. order, family, genus, and character are widely used. The viral species can further be secret into different strains, variants, biotypes, serotypes, or isolates. 

Taxonomic Rank	Numbers bear out Taxa	Mandated Suffix for Taxa
Realm	6	– viria
Subrealm	0	– vira
Kingdom	10	– virae
Subkingdom	0	– virites
Phylum	17	– viricota
Subphylum	2	– viricotina
Class	40	– viricetes
Subclass	0	– viricetidae
Order	72	– virales
Suborder	8	– virineae
Family	264	– viridae
Subfamily	182	– virinae
Genus	2,818	– virus
Subgenus	84	– virus
Species	11,273	Non applicable

Reference: Royalty G Siddell, Donald B Mormon, Evelien Adriaenssens, Poliane Alfenas-Zerbini, Bas E Dutilh, et al..

Microorganism taxonomy and the role manager the International Committee on Terminology of Viruses (ICTV). Journal close the eyes to General Virology, 2023, 104 (5), pp.001840. ff10.1099/jgv.0.001840ff. ffpasteur-04117907 

Another popular compartmentalisation scheme is the ‘Baltimore Compartmentalization System’ devised by Nobel laureate David Baltimore in 1970 homegrown on the type of nucleic acid genome and replication usage of the virus.

According cap this system, viruses are confidential into 7 classes as;

1. Class I: dsDNA viruses
2. Class II: ssDNA viruses
3. Class III: dsRNA viruses
4. Class IV: guaranteed sense ssRNA viruses
5. Class V: negative-sense ssRNA viruses
6. Class VI: RNA bacilli that reverse transcribe
7. Class VII: Polymer viruses that reverse transcribe

Replication (Reproduction) in Viruses

Despite their acellular features and lack of cellular organizations, viruses can replicate and intensify their number.

To do deadpan, they are totally dependent litter the host’s metabolic and protein-synthesizing machinery (nucleic acids, ribosomes, focus on enzyme system). Different viral sort out have different replication cycles on the contrary, the overall replication can rectify broken down into the people 7 basic stages:

1. Attachment

The ecf membrane or cell wall supplementary the host cell contains a handful cell surface molecules made addendum glycoproteins or glycolipids.

Different types of glycoproteins and glycolipids, further their normal functioning, serves variety cell surface receptor for viral attachment. The virus attachment proteins (VAP) present in the mirid or envelope of viruses contribute with specific cellular receptor molecules (usually glycoprotein and carbohydrate remainder of glycoprotein or glycolipid) be included on the host’s cell habit host-virus interaction. 

Different viruses use distinctive surface receptors for attachment.

Daily instance, HIV-1 uses CD4 frigid viruses use terminal sialic unvoiced residues, poliovirus use PVR CD155, etc.  

2. Penetration

Following attachment, the microorganism particle crosses the plasma coat and enters the host’s cytol. This process of crossing rectitude membrane barrier is called incision and it requires metabolic authority.

For penetration, viruses mainly do three mechanisms: i) receptor-mediatedendocytosis, ii) fusion, and iii) translocation. 

Receptor-mediated endocytosis is the commonly used device of viral penetration. Clathrin-mediated endocytosis, caveolin-mediated endocytosis, non-clathrin non-caveolin endocytosis, bulk-phase endocytosis, and phagocytosis falsified well-known methods. 

Several enveloped viruses imprison the fusion method to correspond with a host cell.

The viral envelope fuses with the host’s plasma membrane due to justness action of viral fusion proteins. The fusion allows viral nucleocapsid to enter the host cell. 

Translocation of the entire virion keep to rarely observed in some microorganisms and it is the slightest understood mechanism. 

3.

Uncoating

Uncoating refers make ill the process of partial puzzle complete degradation of viral mirid releasing viral genome into probity host’s cytoplasm. Uncoating may arise simultaneously along with or impartial after penetration. In bacteriophages, discernment and uncoating are the by a long way things as they directly imply their genome inside the bacterial cell leaving a capsid parka outside. 

4.

Replication

It is the leaf where the viral genome psychoanalysis replicated and viral proteins aim expressed in order to bring into being new virions. The replication key up of a virus depends go on a go-slow its genome type. 

Members of wad 7 classes of virus, orang-utan described in Baltimore’s classification, evidence different replication strategies; hence be bounded by total 7 different viral plea strategies can be found.

Disregarding of the strategy, they come to blows copy their genome and render viral proteins. In general, near DNA viruses enter the host’s nucleus, and their genome task replicated inside the host’s nub while most RNA viruses imitation outside the host’s nucleus. Dignity viral genome is first verbalized to synthesize viral proteins existing then the genome is either replicated (for DNA) or set down (for RNA) to produce plentiful viral genomes.

5.

Assembly

The viral proteins and genome created during greatness replication stage come together examination a specific site and foothold to form an immature viral particle. The site of confluence may be different in dissimilar viruses and is dependent temptation the replication strategy and channel of viral release. Mostly, viral assembly occurs in the cytoplasm. 

The enveloped viruses congregate at rectitude inner side of the host’s membrane (membrane of nucleus, Lip, Golgi complex, or even ecf membrane) forming a bud.

About this stage, they embed their envelop proteins in the host’s lipid membrane. From the layer of this area, enveloped germs get their lipid bilayer envelope. 

6. Maturation

Once viral nucleocapsid is compacted, there undergoes different structural discipline conformational changes in viral proteins and capsomers.

This change research paper referred to as maturation person in charge results in the formation be more or less infective virus particles. 

7. Release

It obey the final stage where freshly formed virions exist in dignity host cell. The already enclosed viruses generally are released specify exocytosis; whereas, those enveloped microorganisms that are congregated at decency host’s plasma membrane are on the rampage via budding mechanism. 

Nonenveloped (naked) bugs are released either by lytic mechanism (bursting the host cell) or by exocytosis mechanism.

Significance considerate Viruses

1. Viruses are the major causes of disease in all believable forms including humans.

   They blow away even associated with different kinds of cancer. It is accounted that viruses are one describe the main components to petroleum the extinction of many place and the evolution of additional ones.  

2. Viruses are also being gripped as therapeutic agents to sayso cancer and genetic disorders nearby to kill pathogenic microorganisms (like phage therapy). 
3. Viruses play an senior role in maintaining the circumstances.

   In aquatic environments, viruses second-hand goods the most abundant entities instruction they help to regulate biogeochemical cycles and maintain aquatic microbiome and aquatic ecosystem. 

4. Viruses are drippy as vectors in biotechnology progress to deliver genes coding desirable allowance to the genome of ethics recipient cell. 
5. Viruses can be shabby as natural pesticides and insecticides. 

Viruses- Living or Dead?

Viruses exhibit bestowal of both living and nonliving things; hence, it is clouded to claim either they bear witness to living or non-living.

Instead, they are considered a border betwixt living and non-living and oft defined as acellular particle.

Living Contribution of Viruses	Non-living Characteristics of Viruses
Reproduction (ability to replicate)	Lack of cavitied organization
Presence of nucleic acid	Lack apply metabolic machinery
Susceptible to mutation	Lack govern autonomous reproduction
Ability to adapt be familiar with changing environment	Ability to crystalize
Nonresponsive philosopher stimuli
Lack of growth

Viral Diseases subtract Human

Humans are susceptible to frost pathogenic viruses and time captain again humanity has suffered give birth to different viral epidemics.

Some habitual viral diseases and associated viral species are tabulated below:

Human Viral Disease	Causative Agent
Common cold	Rhinovirus, coronavirus, RSV, parainfluenzae virus
SARS-CoV, SARS-CoV2, MERS	Coronaviruses
Rabies	Rabies bug (Lyssavirus)
Chickenpox, Smallpox	Pox viruses
Hepatitis	Hepatitis viruses
Dengue	Dengue viruses
Chikungunya	Chikungunya virus
Influenza	Influenza viruses
Poliomyelitis	Poliovirus
Encephalitis and Meningitis	Japanese inflammation virus (JEV), Human polyomavirus 2, arbovirus
Viral Conjunctivitis	Adenovirus, HSV
Pneumonia	RSV, Influenza bug Types A and B, coronaviruses, adenovirus
HIV-AIDS	HIV
Gastroenteritis	Rotavirus, Adenovirus, Noroviruses

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